Rotary sprinklers are well known in the irrigation field and usually comprise a nozzle that ejects a stream of water as it is rotated about its longitudinal axis. Some such sprinklers are known as part-circle sprinklers and include nozzles which oscillate back and forth between two arc limits to water only a portion of a circle. Accordingly, part-circle sprinklers include some type of reversible drive which reverses the direction of nozzle rotation at each of the arc limits. the assignee of the present invention, The Toro Company, has made and sold sprinklers of this type known as the SUPER 600.RTM. and SUPER 606.TM..
U.S. Pat. No. 3,107,056 discloses one type of reversible drive used in prior art part-circle sprinklers. In this drive, a unidirectional input gear is part of a gear train that includes two different gear sets, having unequal numbers of gears, which end in two drive gears that rotate in opposite directions. The gear sets and input gear are mounted on a support yoke which is pivotable about the axis of the input gear. A shiftable trip arm oscillates the yoke between one of two positions in which one or the other of the drive gears has been swung into engagement with an internal driven gear carried on the nozzle. Thus, the nozzle rotates in different directions depending upon which drive gear is in engagement with the internal gear.
While the above-noted drive has been widely used in part-circle sprinklers, it has one major disadvantage, namely, the gear set having the larger number of gears engages the driven gear with considerably more force than the other gear set. In some cases, this additional force is so great as to make drive reversal difficult or impossible, since it is merely the trip arm carried by the nozzle which acts on the support yoke to shift the yoke from one orientation to the other. Therefore, some sprinklers of this type will not always reliably shift directions.
U.S. Pat. No. 4,568,024 to Hunter recognizes the problem described above and attempts to solve it in the prior art structure by using gears in the gear sets having different pressure angles on the different faces thereof. The apparent theory there is that by suitably controlling the pressure angle on the gear teeth, the drive gear can be made to slip out of the internal gear more easily, thereby allegedly solving the disengagement problem. However, as will be explained more fully in the Detailed Description, Applicant does not believe this approach truly solves the disengagement problem because it treats the symptoms of the problem without understanding or discovering the basic cause of the problem. Moreover, even if there were some validity to the pressure angle approach, it is a relatively difficult one to execute properly on a mass-produced basis because of the relatively close tolerances required in the manufacture of the pressure angles on the teeth of the various gears, which are after all quite small.